The ability to make accurate and precise measurements of antibody-mediated virus neutralization is important for the evaluation of mechanisms of neutralization and for the assessment of antibody responses elicited by immunization. For HIV-1, the use of primary virus isolates and human target cells, such as PBMC, is an in vitro assay system that is believed to measure physiologically relevant virus neutralization. Using this assay, preclinical antibody responses generated by vaccine candidates were screened and measured.